Although applicable to any fibre composite components, the present invention and the problem on which it is based will be described in detail below with reference to planar carbon-fibre-reinforced plastics material (CFRP) components (also referred to as fibre composite components), for example skin shells of an aircraft.
It is generally known to reinforce CFRP skin shells using CFRP stringers, in order to withstand the high loads in the aircraft region with as low an additional weight as possible. In this case, various stringer types are used, for example T, Ω or I stringers.
In the following, stringers having a hollow profile will be considered, for example stringers having a cap profile, Ω profile and the like. The term hollow profile relates to a stringer profile, the inner faces of which, together with a portion of a fibre composite component to be reinforced, form a cavity or interior. Hereinafter this interior is referred to as a receiving region.
This will be illustrated in FIG. 1 by way of a schematic perspective view of a reinforced fibre composite component.
Two reinforcing elements 3 are applied to a fibre composite component portion 2, for example a skin shell or a skin laminate of an aircraft, of the reinforced fibre composite component 1. The reinforcing elements 3 are configured as a hollow profile, in this example as a cap profile or Ω stringer. They each comprise two foot portions 13 which as a base form the connecting face to the skin shell. The foot portions 13 are connected to a cover wall 10 via side walls 11 and 12, which in this case project upwards at an angle, the cover wall 10 extending substantially parallel to the foot portions 13. The inner faces of the cover wall 10, side wall 11 and 12 and the portion of the fibre composite component portion 2 which is located therebelow define the above-mentioned interior, which is referred to as a receiving region 9. In this case, the regions where the reinforcing elements 3 are mounted are each characterised as a moulding portion 8. In this case, after completed mounting on the fibre composite component portion 2, the left-hand reinforcing element 3 is also provided with a pressure element 4 which is inserted into the receiving portion 9.
When producing fibre composite components of this type from fibre composite plastics materials, it is necessary during the curing process to compact the composite of fibres and matrix materials in order to avoid air inclusions and to be able to achieve a particular fibre volumetric content in the cured skin laminate.
Fibrous semi-finished products are to be understood to mean woven fabrics, non-woven fabrics and fibre mats. These are provided with a matrix, for example an epoxy resin, and then cured, for example in an autoclave.
Owing to the mostly planar configuration of the fibre composite components 1, this compaction of the fibrous semi-finished products or fibres during the curing process is generally effected by the use of differential pressure. For this purpose, a vacuum construction is created, which is advantageously produced by a plastics material film which at its edges is hermetically bonded to the mould. The mould and the film thus form a hermetically sealed space in which the fibrous semi-finished product is enclosed. By removing the medium inside this hermetically sealed space by suction, a relative overpressure is achieved outside the vacuum construction, whereby the composite of fibres and matrix can be pressed during the curing process. Alternatively or in addition, the force for compacting the fibre/resin composite can be increased by a pressure increase outside the vacuum construction.
The production of fibre composite components requires a certain pressure during the curing process in order to compact the skin laminate. This pressure can be applied or transferred to the component by various pressure elements. As mentioned above, a pressure element of this type is shown by reference numeral 4 in FIG. 1. In this case, the pressure element 4 is also required in particular to exert pressure on the face of the skin laminate which is covered by the reinforcing element 3, between the foot portions 13 of the reinforcing element 3, in order to compact this portion.
It is often necessary to position and fasten the pressure element in a precise manner relative to the semi-finished product and the curing device. Where applicable, component parts must be received, transported and positioned as a unit together with the pressure elements, for example by means of a gripper. For this purpose a fastening device is required which fastens component parts and pressure elements to one another and which can be released again at the latest at the end of the production process. This relates to all types of fibre composite production methods, in particular prepreg and dry fibre methods, as well as similar production methods in which, for example, one or the two parts of the component may consist of other materials, for example a light metal. Particularly in the case of processes involving cap profiles, the interior or receiving region must be equipped with a pressure element in order to compact the inner walls and in particular the skin laminate of the portion of the fibre composite component which is located below the receiving portion 9 (FIG. 1).
Until now, the pressure elements have either been received, transported and then positioned relative to one another separately from the components to be pressurised, or a common integration of a so-called subelement (fibre composite profile, reinforcing element) and a pressure element takes place in the vacuum construction, although this is only possible in the case of very small components, when the subelement and pressure element can be clamped together at their end faces, for example. For large and very long component parts, this is only possible at considerable additional cost. The separate integration of the pressure elements and subelements of the fibre composite component during the curing process results in positioning inaccuracies and therefore geometrical differences of the subsequent component. The pressure elements are at risk of being displaced during the process. In the worst case, they may be clamped in the component and laminated in.
Until now the pressure elements have therefore been positioned and attached using gravity and friction and/or using one- or two-sided adhesive tapes, using which said elements can be fixed inside and/or in/on the curing device. This method leads to the drawback that these elements can sometimes no longer be released after curing, the pressure elements therefore having to remain in the component after curing, possibly leading to rejection of the component.
The following methods are also known:
1) The first side of the subsequent fibre composite component in the form of the fibrous semi-finished product is laid in the curing device. The pressure element is positioned on said side before the cap element (reinforcing element) is put on. A drawback in this case is that the pressure element can be displaced and/or clamped between the two parts of the component during the curing process.
2) The pressure element is laid in the cap profile and held manually while the cap element is being positioned. However, this is only possible if the dimensions of the component are small. This method is not possible in the case of relatively large components, since many people and coordinated action are required for this purpose. In this case, the pressure element may fall out of the cap element, and there may therefore be a risk of damage. A further risk is that of the pressure element being clamped between the skin laminate (first side of the subsequent fibre composite component) and the second part (reinforcing element).
3) When the component is configured with a large length and a constant internal cross-section, it is possible to draw in the pressure element after placing the cap element on the skin laminate. However, this method can only be used if the lengths of the component do not exceed particular lengths dependent on the material of the pressure element. In this case, however, the cap element must have a constant internal cross-section over the entire length.
4) The pressure element is attached to the skin laminate using adhesive tapes. The pressure element can no longer be removed in the case of relatively large lengths of the cap element, since the bond between the pressure element and the component is reinforced by the pressure differential.
5) Attachment using an adhesive is not possible for all types of pressure element, since these must have a surface which is anti-adhesive with regard to the material of the matrix. Owing to these surfaces, low-viscosity adhesives roll off, preventing the application of adhesive.
6) When hollow components are produced from fibre composite materials comprising pre-impregnated fibres in the co-bonding method, the pressure element can be attached or fastened with positive locking by means of an adhesive film. For this purpose, the pressure element is integrated into the cured component (reinforcing element) and prevented from falling out by a continuous adhesive film. However, this can only be used for a previously cured component and fibre composite components made of pre-impregnated fibres. In addition, the cured component has a higher weight owing to the additional adhesive film.
DE 10 2007 061 431 A1 discloses a method for reinforcing a fibre composite component, in which a reinforcing element is received by a receiving portion of a vacuum mat and applied in a sealing manner to a fibre composite component to be reinforced, to form a moulding portion. The vacuum mat can also receive stringers having a hollow profile, but then pressure elements are also required since the vacuum mat is not adapted for interiors of the stringers.
DE 10 2008 032 834 discloses a method for positioning a tubular mould core in the receiving region of a reinforcing element. Means for positioning the mould core in the receiving region with positive locking and/or frictional engagement are provided at the reinforcing element. The means for positioning the mould core are provided, for example, as clip-shaped undercuts in the transition region between the foot portions and the side walls of the reinforcing element.
EP 2 159 039 discloses a method for producing composite structures, the composite material being placed on a mould body for shaping. For positioning the composite material, magnets are provided in the mould body. Owing to the fact that the composite material comprises fibres made of a magnetic material, the composite material can be placed and fastened accurately on the mould body.